Computer simulation of defect motion in model normal and ‘fast-ion’ conductors I. NaCl and AgCl

Abstract

The molecular dynamics simulation technique has been used to study Frenkel defects in crystalline NaCl and AgCl. Partial radial distribution functions, mean square displacements and the velocity autocorrelation function with its Fourier transform have been calculated for cation and anion species. Because the minimum Frenkel defect concentration that can be introduced into the molecular dynamics sample is 1/108, the initial configurations represent a non-equilibrium state and their evolution in time provides an opportunity to study mechanisms of defect migration that are involved in the recombination of Frenkel defects. Direct interstitial jumps, collinear interstitialcy migration, and [110] vacancy jumps have all been observed. The time dependence of the mean square displacements, and particularly the ratio of the square of second moment to the fourth moment of the self-correlation function, are sensitive indicators of the occurrence of defect jumps. Plots of the individual ion displacements in various time windows reveal which ions are in motion, while the projections of the cation and anion sublattices in the simulation cell give a detailed picture of the motion of the defects.